For the more than 10 million US women using oral contraceptives (OCs), clear evidence regarding the health effects of OCs is a major public health concern. Several recent studies suggest that current OC formulations may act to suppress peak bone mass accrual in young women. However, this possibility has not been fully evaluated, particularly with regard to OC use among adolescents, the group most actively gaining bone. This application is focused on a thorough evaluation of the impacts of OC use on bone in young women. We include consideration of different skeletal sites, varying durations of use, and differing OC estrogen content. Importantly, this would be the first prospective study to our knowledge to document changes in bone mineral density (BMD) after OC discontinuation. We propose a prospective epidemiologic study of the effects on bone of new and ongoing OC use and of OC discontinuation in a cohort of adolescent and young adult women. The cohort (n=600) will be recruited from a defined population of HMO enrollees between the ages of 14 and 30, with 300 participants aged 14-18 years and 300 aged 19-30. Each age group will include 200 OC users and 100 comparison women. We will evaluate possibly different effects from a low-dose (20 mcg EE) and a higher-dose (35 mcg EE) formulation. The HMO's automated pharmacy and membership files will be used to select women who are new and ongoing users of OCs and to randomly select comparison women who are not using hormonal contraception, frequency matching on age and clinic. Participants will be followed longitudinally for 24-36 months. The primary outcome, bone mineral density, will be measured every 6 months at selected anatomic sites (hip, spine, whole body) using dual-energy x-ray absorptiometry. In our work to date investigating the association between injectable contraception and BMD in adolescents and young adult women, we have achieved excellent participant recruitment and retention with similar methods and have developed relevant measurement and analytic strategies. OC use is highest among sexually active adolescents and women in their 20s. These are also women who are actively gaining bone. Given the increasing public health importance of osteoporosis and the prevalence of OC exposure at a crucial stage of growth, focused evaluation of the potentially adverse effects of OC use on the skeletal health of these young women--and their possible amelioration upon cessation of this exposure--is essential.